线程同步的三个例子

synchronized 用法有两种，简单区分：

1. synchronized 函数 -- 程序只有一个类，把实际操作的函数从 run()方法 独立出来，在该函数定义前面加一个 synchronized 关键字（private synchronized void buy()）
2. synchronized 代码块 -- 程序有两个类以上，把实际要操作的变量所在对象找到

synchronized (account){
            // 判断有没有钱
      if (drawingMoney > account.money) {
          System.out.println(Thread.currentThread().getName()+"钱不够了，取不了");
          return;
      }

      try {
          Thread.sleep(1000);
      } catch (InterruptedException e) {
          e.printStackTrace();
      }

       // 卡内余额
      account.money = account.money - drawingMoney;
      nowMoney = nowMoney + drawingMoney;
      System.out.println(account.name+"余额为："+account.money);
//    System.out.println(Thread.currentThread().getName());
      System.out.println(this.getName()+"手里的钱"+nowMoney);
}

三个例子

1.抢票程序（线程不安全版）

package demo04_synchronized;


// 不安全地买票
// 线程不安全，有负数
// WARNING！！
// 每个线程在自己的工作内存交互，内存控制不当会造成数据不一致
public class UnsafeBuyTicket {
    public static void main(String[] args) {
        BuyTicket station = new BuyTicket();

        new Thread(station, "苦逼的我们").start();
        new Thread(station, "牛逼的你们").start();
        new Thread(station, "可恶的黄牛党").start();
    }

}

class BuyTicket implements Runnable{

    // 票
    private int ticketNums = 10;
    boolean flag = true; // 外部停止方式


    @Override
    public void run() {
        // 买票
        while (flag){
            buy();
        }
    }

    private void buy(){
        // 判断是否有票
        if (ticketNums <= 0) {
            flag = false;
            return;
        }

        // 延时设置
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        // 买票
        System.out.println(Thread.currentThread().getName()+
                "买到第"+ticketNums--+"张票");
    }

}

抢票程序改进版

只是在 buy() 函数前加了个 synchronized

package demo04_synchronized;


// 不安全地买票
// 线程不安全，有负数
// WARNING！！
// 每个线程在自己的工作内存交互，内存控制不当会造成数据不一致
public class UnsafeBuyTicket {
    public static void main(String[] args) {
        BuyTicket station = new BuyTicket();

        new Thread(station, "苦逼的我们").start();
        new Thread(station, "牛逼的你们").start();
        new Thread(station, "可恶的黄牛党").start();
    }

}

class BuyTicket implements Runnable{

    // 票
    private int ticketNums = 10;
    boolean flag = true; // 外部停止方式


    @Override
    public void run() {
        // 买票
        while (flag){
            buy();
        }
    }

    private synchronized void buy(){
        // 判断是否有票
        if (ticketNums <= 0) {
            flag = false;
            return;
        }

        // 延时设置
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        // 买票
        System.out.println(Thread.currentThread().getName()+
                "买到第"+ticketNums--+"张票");
    }

}

2.银行取钱例子（线程不安全版）

package demo04_synchronized;


// 不安全的取钱
// 两个人去银行取钱，账户
public class UnsafeBank {
    public static void main(String[] args) {
        Account account = new Account(100, "结婚基金");
        Drawing you = new Drawing(account, 50, "you");
        Drawing gf = new Drawing(account, 100, "gf");

        you.start();
        gf.start();

    }
}

// 账户
class Account{
    int money;  // 余额
    String name; // 卡名

    public Account(int money, String name) {
        this.money = money;
        this.name = name;
    }
}


// 银行：模拟取款
class Drawing extends Thread{
    Account account; // 账户
    // 取了多少钱
    int drawingMoney;
    // 手里余额
    int nowMoney;

    public Drawing(Account account, int drawingMoney, String name){
        super(name);
        this.account = account;
        this.drawingMoney = drawingMoney;
    }

    // 取钱
    // synchronized 默认锁的是 this
    @Override
    public void run() {
        // 判断有没有钱
        if (drawingMoney > account.money) {
            System.out.println(Thread.currentThread().getName()+"钱不够了，取不了");
            return;
        }

        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        // 卡内余额
        account.money = account.money - drawingMoney;
        nowMoney = nowMoney + drawingMoney;
        System.out.println(account.name+"余额为："+account.money);
//        System.out.println(Thread.currentThread().getName());
        System.out.println(this.getName()+"手里的钱"+nowMoney);

    }
}

改进版（线程安全版）

package demo04_synchronized;


// 不安全的取钱
// 两个人去银行取钱，账户
public class UnsafeBank {
    public static void main(String[] args) {
        Account account = new Account(100, "结婚基金");
        Drawing you = new Drawing(account, 50, "you");
        Drawing gf = new Drawing(account, 100, "gf");

        you.start();
        gf.start();

    }
}

// 账户
class Account{
    int money;  // 余额
    String name; // 卡名

    public Account(int money, String name) {
        this.money = money;
        this.name = name;
    }
}


// 银行：模拟取款
class Drawing extends Thread{
    Account account; // 账户
    // 取了多少钱
    int drawingMoney;
    // 手里余额
    int nowMoney;

    public Drawing(Account account, int drawingMoney, String name){
        super(name);
        this.account = account;
        this.drawingMoney = drawingMoney;
    }

    // 取钱
    // synchronized 默认锁的是 this
    @Override
    public void run() {

        // 要判断锁住要操作的对象是谁？就是要增删改查的变量在哪，这里是account，而不是this
        synchronized (account){
            // 判断有没有钱
            if (drawingMoney > account.money) {
                System.out.println(Thread.currentThread().getName()+"钱不够了，取不了");
                return;
            }

            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            // 卡内余额
            account.money = account.money - drawingMoney;
            nowMoney = nowMoney + drawingMoney;
            System.out.println(account.name+"余额为："+account.money);
//        System.out.println(Thread.currentThread().getName());
            System.out.println(this.getName()+"手里的钱"+nowMoney);

        }
        }
}

3.线程入队列例子（线程不安全版）

package demo04_synchronized;


import java.util.ArrayList;
import java.util.List;

// 线程不安全的集合
public class UnsafeList {
    public static void main(String[] args) {
        List<String> list= new ArrayList<String>();
        for (int i = 0; i < 10000; i++) {
            new Thread(()->{
                list.add(Thread.currentThread().getName());
            }).start();
        }
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(list.size());
    }
}

改进版（线程安全版）

package demo04_synchronized;


import java.util.ArrayList;
import java.util.List;

// 线程不安全的集合
public class UnsafeList {
    public static void main(String[] args) {
        List<String> list= new ArrayList<String>();
        for (int i = 0; i < 10000; i++) {

            synchronized (list) {
                new Thread(()->{
                    list.add(Thread.currentThread().getName());
                }).start();
            }
        }
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(list.size());
    }
}